1. Field of the Invention
The present invention relates to path timing detection in an adaptive array antenna system and, more particularly, to a path timing detection method, a path timing detection apparatus and an adaptive array antenna system in which code division multiple access (CDMA) signals are received by an array antenna and path timings are detected by using signals obtained by multibeam reception.
2. Description of the Related Art
Code division multiple access (CDMA) has the potential for increasing the number of subscribers in a communication system and is therefore considered a promising wireless access method for mobile communication cellular system. However, user signals access a base station simultaneously with a desired user signal and interfere with the desired user signal. A technique using an adaptive array antenna is used to eliminate such interference in a spatial region.
In an adaptive array antenna system, signals are received by a plurality of antennas, a directional beam is formed by combining the received signals in a complex number weighted combination manner, a desired user signal is thereby received while interference of other user signals is suppressed.
In a CDMA mobile communication system, there is a need to ascertain, in a reception and demodulation section, timings of paths included in a multipath which occurs from a propagation environment for a signal, and which are necessary for reception and demodulation of the signal, and a path timing detection section is therefore provided. A path timing detection section in an adaptive array antenna system detects a plurality of path timings before reception and demodulation and notifies a reception and demodulation section of the detected timings. The reception and demodulation section despreads received signals by the notified path timings, forms a user-specific directional beam from the signals, and combines path signals (by rake combining) to finally obtain demodulation results.
In a multiple antenna system such as an adaptive array antenna system, the ratio of signal power to interference noise power (signal to interference noise ratio: SINR) per antenna decreases in proportion to the number of antennas. Therefore a deterioration in path detection characteristics, particularly a reduction in SINR occurs if path timing detection is performed by using a received signal from only one antenna. As a method devised by considering this problem, a method of performing path timing detection by using signals received with a plurality of antennas is known. An example of a system for performing path timing detection by this method is one described in Japanese Patent Laid-open Publication No. 2000-22587 “Spread Spectrum Signal Receiving Method and Receiver”. This system is arranged to improve detection characteristics in such a manner that delay profiles are formed on an antenna-by-antenna basis and are combined to obtain a smoothed delay profile.
FIG. 7 is a diagram showing an example of a configuration of a conventional path timing detection apparatus and adaptive array antenna system.
The conventional adaptive array antenna system has a path timing detection section 101 and a reception and demodulation section 106. The path timing detection section 101 has means for detecting path timings by receiving code division multiple access (CDMA) signals by an array antenna, i.e., a sliding correlator 102, a per-antenna delay profile generation means 103, a delay profile combining means 104, and a path timing detection means 105.
The sliding correlator 102 despreads signals from antennas at a resolution of 1/NR (NR: an integer equal to or larger than 1) of the chip period over a plurality of chip timings, and outputs despread signal sequences. The per-antenna delay profile generation means 103 averages in a certain period each of the signal sequences from the sliding correlator 102 to generate a delay profile with respect to each antenna. The delay profile combining means 104 combines the delay profiles generated by the means 103 to generate one resultant delay profile. The path timing detection means 105 detects, on the basis of the one resultant delay profile, a plurality of path timings to be used in the reception and demodulation section 106.
FIG. 8 is a diagram for explaining the operation of the conventional path timing detection means 105. For detection of path timings, a detection method of selecting chip timings with a high level from one delay profile from the delay profile combining means 104 is used. Since narrow-band frequency band limitation is ordinarily performed in the CDMA system, a signal in one path appears by spreading over a plurality of chip timings. A method is therefore adopted in which, in the process of selecting chip timings with a high level one after another from a delay profile, the path timing detection means 105 sets a chip timing interval 121 centered on a chip timing already selected as a path timing (hereinafter referred to as “minimum selection timing interval”), and does not select chip timings less than the minimum selection timing interval 121 in path timing detection subsequently performed.
That is, referring to FIG. 8, if NR is 4 and if the minimum selection timing interval 121 is ±4 chip timings (one chip period), a chip timing 6 with a highest level is selected as a first path timing. Chip timings less than the minimum selection timing interval 121 are excluded in path timing detection subsequently performed. That is, chip timings 3, 4, 5, 7, 8, and 9 on the opposite sides of the chip timing 6 are excluded. Next, a chip timing 13 is selected as a second path timing, and chip timings 10, 11, 12, 14, 15, and 16 on the opposite sides of the chip timing 13 are excluded. In this processing, chip timing 5, for example, exists with a level higher than that with which chip timing 13 exists, but it is regarded as a spread of the first path and is not selected. The same path timing detection processing is subsequently repeated. An interval defined by ±3 or ±4 chip timings is suitably set as minimum selection timing interval 121 when NR=4.
The reception and demodulation section 106 has means for receiving code division multiple access (CDMA) signals by the array antenna and demodulating path signals by detected path timings, i.e., L number of path receiving means 107-1 to 107-L corresponding to the number of multipath propagation paths, and one combiner 111. The path receiving means 107-1 to 107-L have correlators 108-1 to 108L, beam formers 109-1 to 109-L, rake combining weighting means 110-1 to 110-L, and antenna weight control means 112-1 to 112-L.
The correlators 108-1 to 108-L despread spread signals by path timings detected by the path timing detection section 101 in correspondence with the paths. The beam formers 109-1 to 109-L form antenna directional beams from outputs of the correlators 108-1 to 108-L in correspondence with the paths by using user-specific antenna weights adaptively generated. The rake combining weighting means 110-1 to 110-L weight beam outputs corresponding to the paths to make compensation for carrier phase variations and to maximize the SINR after combining (to achieve maximum-ratio combination). The antenna weight control means 112-1 to 112-L compute antenna weights by using the despread signals from the correlators 108-1 to 108-L.
As antenna weight control means 112-1 to 112-L, a method of computing antenna weights such that beams are directed to paths on the basis of estimation of the arrival angles of the paths and a method of using an adaptive algorithm such as that for minimum-mean-squared error (MMSE) control are known. For example, an MMSE method is described in Japanese Patent Laid-open Publication No. 2002-77008 “Adaptive Antenna Receiver”. The combiner 111 adds together outputs from the rake combining weighting means 110-1 to 110-L and outputs high-quality demodulation results obtained by combining the paths.
The path timing detection apparatus in the conventional adaptive array antenna system shown in FIG. 7 is arranged to improve detection characteristics by improving path timing discrimination in such a manner that level combining (amplitude or power combining) of delay profiles corresponding to respective antennas is performed to obtain a smoothed delay profile.
However, this method only reduces the spread of each of the peak level and the noise level of delay profiles with respect to paths and does not immediately improve the SINR (the difference between the peak level and the noise level). In particular, if the number of antenna elements included in the array antenna is increased, the SINR per antenna is reduced and peaks in paths do not appear distinctively. Thus, there is a limit to the improvement in characteristics based on delay profile combining.